A. Technical Field
The present invention relates to: a process for producing a water-absorbent resin which is utilized favorably for various uses such as sanitary articles (e.g. disposable diapers, sanitary napkins) and water-retaining agents for soil, wherein the water-absorbent resin is produced by polymerizing an aqueous solution of water-absorbent resin-forming monomers; products (hydropolymer and water-absorbent resin) from this process, and sanitary articles comprising the water-absorbent resin.
B. Background Art
In recent years, water-absorbent resins are widely utilized for various uses such as sanitary articles (e.g. disposable diapers, sanitary napkins, adult incontinent products) and water-retaining agents for soil and are produced and consumed in large quantities.
Particularly in the uses for the sanitary articles (e.g. disposable diapers, sanitary napkins, adult incontinent products), the tendency is toward increasing the amounts of water-absorbent resins and decreasing the amounts of pulp fibers in order to render products thin, and the water-absorbent resins are desired to have large absorption capacities under loads, while the water-absorbent resins are used in so large quantities per sheet of the sanitary articles that the water-absorbent resins are desired to cost low. Therefore, in the production line of the water-absorbent resins, it is desired to reduce energy consumption and waste matter emission and to thereby establish a reasonable production process.
Various polymerization processes have been attempted so far in which, for example, when aqueous solution polymerization of water-absorbent resin-forming monomers is carried out, a dried water-absorbent resin is obtained at one stroke by carrying out polymerization in a high monomer concentration or initiating polymerization at a high temperature to thereby vaporize water by a heat of polymerization or heating, for the purpose of rendering costs so low as to enhance the performance/cost ratio of the water-absorbent resin.
JP-A-071907/1983 (Arakawa Kagaku) and JP-A-018712/1984 (Arakawa Kagaku) disclose processes in which dry solids of water-absorbent resins are obtained at one stroke by polymerizing an aqueous acrylate salt solution of a concentration higher than 55 weight %. U.S. Pat. No. 4,985,518 (American Colloid) discloses a process in which a dry solid of a water-absorbent resin is obtained at one stroke by polymerizing an aqueous acrylate salt solution of a concentration higher than 30 weight %. JP-A-058208/1980 (Kitani) discloses a process in which polymerization is carried out in the polymerization temperature range of 106 to 160° C. without any crosslinking agent, and in examples thereof a dry solid having a low water content is formed at the end of the polymerization. JP-A-318022/1989 (Mitsubishi Yuka) discloses a process in which a polymer is obtained in almost a dry state by polymerizing an aqueous solution containing a monomer in a concentration of 45 to 80 weight % wherein the monomer has a neutralization ratio of 20 to 50 mol %. However, these processes have demerits in that the resultant water-absorbent resins have high extractable contents for their absorption capacities.
In addition, JP-A-147512/1980 (Sumitomo Chemical Co., Ltd.), JP-A-147809/1981 (Sumitomo Chemical Co., Ltd.), JP-A-275607/1988 (Sanyo Kasei), and JP-A-275608/1988 (Sanyo Kasei) disclose processes in which dried products of water-absorbent resins are obtained at one stroke by supplying aqueous monomer solutions onto heated rotary drums and then scraping off the resultant polymers therefrom. JP-A-165610/1989 (Rohm and Haas) also discloses almost the same process as the above in which a substantially dry solid of a water-absorbent resin is obtained by supplying an aqueous monomer solution onto a heated face. However, these processes also have the demerits in that the resultant water-absorbent resins have high extractable contents for their absorption capacities.
In addition, JP-A-215801/1990 (Mitsubishi Yuka) discloses a process in which polymerization is carried out by spraying into a gas phase an aqueous monomer solution as heated by utilizing a heat of neutralization of the monomer, but, as to this process, it is considered that the polymerization is difficult to control, because the polymerization is completed in about 3 seconds.
The above prior arts were published in or before 1990, but have their respective demerits, therefore it seems that they are not actually carried out.
Thereafter published were arts as directed to enhancement of the performance for the purpose of increasing the performance/cost ratio of the water-absorbent resin. JP-A-175319/1992 (Sanyo Kasei) and JP-A-181005/1999 (Nippon Shokubai Co., Ltd.) disclose attempts to obtain high-performance water-absorbent resins by initiating polymerization at a low temperature and mildly carrying out the polymerization while removing the generated heat to depress the peak temperature to not higher than about 90° C. JP-A-228604/1999 (Nippon Shokubai Co., Ltd.) discloses an attempt to obtain a high performance water-absorbent resin still by initiating polymerization at a low temperature and mildly carrying out the polymerization while removing the generated heat to depress the peak temperature to not higher than about 95° C. or to control the increase of the solid component concentration within the range of 0.2 to 10 weight %. JP-A-067404/1997 (BASF) and U.S. Pat. No. 6,187,828 (BASF) disclose a process in which polymerization is initiated at a low temperature in a cylindrical polymerization reactor and carried out adiabatically, but this process has demerits in that the concentration of the aqueous monomer solution cannot be rendered high, because the heat removal is not carried out, and in that the residence time is long (several hours). All these processes sacrifice the productivity, therefore none of them can avoid high costs.
In addition, recently, “An Efficient Preparation Method for Superabsorbent Polymers” (Chen, Zhao) was reported in the Journal of Applied Polymer Science, Vol. 74, pp. 119-124 (1999). This proposes a low-cost polymerization process comprising the steps of placing an aqueous solution of a monomer concentration of 43.6% and an initiator into a stainless Petri dish, and then immersing this dish into a water bath of 70° C. or 80° C. to carry out polymerization. However, this process has not yet attained an industrially useful level.
In addition, JP-A-045812/1998 (Sekisui Chemical Products) discloses an attempt to inhibit bumping, promote emission of water vapor, and render the water content of the resultant gel low by adding short fibers to an aqueous monomer solution, but this attempt has the demerit of using expensive short fibers which do not contribute to the water absorption.